Encyclopedia of Espionage, Intelligence, and Security

Nuclear Spectroscopy

█ K. LEE LERNER

Nuclear spectroscopy is a powerful tool in the arsenal of scientists and
forensic investigators because it allows detailed study of the structure
of matter based upon the reactions that take place in excited atomic
nuclei. It is a widely used technique to determine the composition of
substances because it is more sensitive than other spectroscopic methods
and can detect the trace presence of elements in an unknown substance that
may only be present on the order of parts per billion. Nuclear
spectroscopic analysis techniques provided forensic investigators with
evidence that linked several of what were eventually to be known as the
Washington area "sniper shootings" in late 2002.

Basic principles.
A number of methods can be used to excite atomic nuclei and then measure
their decaying gamma ray emissions as the atoms return to normal energy
levels (i.e., their ground state). The emissions are then analyzed and
separated into an emission spectrum that is characteristic for each
element. Excitation can be accomplished by colliding nuclei, heavy ion
beams, and a number of other methods, but the fundamental purpose remains
to measure the spectral properties of a sample as a tool to learn
something about the quantum structure of the atoms in the sample.

Like other forms of spectroscopy, the fundamental measurements of nuclear
spectroscopy involve recording the emission or absorption of photons by
atoms. The specific emissions or absorptions reflect the energy levels,
spin states, parity, and other properties of an atom's structure
(e.g., quantized energy levels). A qualitative analysis identifies the
components of a substance or mixture. Quantitative analysis, on the other
hand, measures the amounts or proportions of those components. Because
each element—and each nuclide (i.e., an atomic nucleus with a
unique combination of protons and neutrons)—emits or absorbs only
specific frequencies and wavelengths of electromagnetic radiation, nuclear
spectroscopy is a qualitative test (i.e., a test designed to identify the
components of a substance or mixture) to determine the presence of an
element or isotope in an unknown sample.

In addition, the strength of emission and absorption for each element and
nuclide can allow for a quantitative measurement of the amount or
proportion of the element in an unknown. To perform quantitative tests,
that is, to measure amounts of an element present, the measured spectrum
needs to be narrowed down to analysis of photons with specific energies
(i.e., electromagnetic radiation of a specific wavelength or frequency).
Quantitative computation using Beer's Law is then applied to the
measured intensities of photon emission or absorption. Many other
spectroscopic methods use this technique (e.g., atomic absorption
spectroscopy and UV-visible light spectroscopy) to determine the amount of
a element present.

Nuclear activation analysis.
One of most widely used methods of nuclear spectroscopy used to determine
the elemental composition of substances is Nuclear activation analysis
(NAA). In this type of analysis the goal is to determine the composition
of an unknown substance by measuring the energies and intensities of the
gamma rays emitted after excitation and the subsequent matching of those
measurements to the emissions of gamma rays from standardized (known)
samples. In this regard, neutron activation analysis is similar to other
spectroscopic measurements that utilize other portions of the
electromagnetic spectrum. Infrared photons, x-ray florescence, and
spectral analysis of visible light are all used to identify elements and
compounds. In each of these spectroscopic methods, a measurement of
electromagnetic radiation is compared with some known quantum
characteristic of an atomic nucleus, atom, or molecule. With NAA, of
course, high-energy gamma-ray photons are measured.

Neutron activation analysis involves a comparison of measurements from an
unknown sample with values obtained from tests with known samples.
Depending on which elements are being tested for, the samples are
irradiated with energetic neutrons. The process of radioactivity results
in the emission of products of nuclear reactions (in this case, gamma
rays) that are measurable by instruments designed for that purpose. After
a time (dependent on the duration of radiation) the gamma rays are counted
by gamma ray sensitive spectrometers. Because the products of the nuclear
reactions are characteristic of the elements present in the sample and a
measure of the amounts present, neutron activation analysis is both a
qualitative and quantitative tool. Although NAA usually involves the
measurement of gamma rays emitted from the radioactive sample, more
complex techniques also measure beta and positron emissions.

Nuclear magnetic resonance.
Nuclear magnetic resonance (NMR) is another form of nuclear spectroscopy
that is widely used in medicine and in forensic analysis. NMR is based on
the fact that a proton in a magnetic field has two quantized spin states.
The actual magnetic field experienced by most protons is, however,
slightly different from the external applied field because neighboring
atoms serve to alter it. As a result, a picture of complex structures of
molecules and compounds can be obtained by measuring differences between
the expected and measured photons absorbed. NMR spectroscopy is an
important tool used to determine the structure of organic molecules.

When a group of nuclei are brought into resonance—that is, when
they are absorbing and emitting photons of similar energy (electromagnetic
radiation, e.g., radio waves, of similar wavelengths)—and then
small changes are made in the photon energy, the resonance must change.
How quickly and to what form the resonance changes allows for the
non-destructive (because of the use of low-energy photons) determination
of complex structures. This form of NMR is used by physicians as the
physical and chemical basis of a powerful diagnostic technique termed
Magnetic Resonance Imaging (MRI). MRI can also be used for noninvasive
examinations for concealed substances or implanted objects.